1. FIELD OF THE INVENTION
The invention relates, in general, to non-volatile electronic memories and more particularly to a method and apparatus for reducing fatigue on a ferroelectric device in a memory cell by altering the duty cycle of signals used for switching the ferroelectric material polarization.
2. STATEMENT OF THE PROBLEM
It is known that ferroelectric materials are capable of retaining a polarization which can be used to store information in a non-volatile memory. For example, if a strong enough electric field is placed across a ferroelectric capacitor, when the electric field is removed, a polarization in the direction of the field remains within the ferroelectric material of the capacitor. If the electric field is placed across the same capacitor in the opposite direction, when the field is removed, a polarization in the opposite direction remains within the ferroelectric capacitor. Electronic sense circuits have been designed to associate the remnant polarization in one direction with a logic "1" state, and remnant polarization in the opposite direction with a logic "0" state. A useful circuit for this purpose is disclosed in co-pending U.S. patent application Ser. No. 08/092,233, filed on July 15, 1993 by Mihara et al. These sense circuits are similar to, and adapted from, those found in more traditional DRAM devices. In sensing the polarization state of a ferroelectric capacitor, the polarization is altered, therefor the sense circuit also includes a capability to re-write the original polarization state back to the memory cell after sensing the present state.
Ferroelectric RAM structures, like traditional memory structures, include memory cells arranged in rows and columns. Each memory cell typically includes at least one transistor having a gate, source, and drain, and a capacitor having a pair of plate electrodes. Each memory cell further includes plate lines connected to one plate electrode of the capacitor in each cell, bit lines connected to the other plate of the capacitor through the transistor, and word lines connected to the control gate of the transistor. The transistor acts as a switch controlled by its gate, to connect the capacitor to the plate line.
All known prior designs for memories which utilize ferroelectric devices share a common problem in that the frequent switching of the polarization of a ferroelectric material tends to fatigue the ferroelectric material. Frequent switching of the ferroelectric material, over time, is believed to exacerbate ionic migration of point charge defects within the ferroelectric crystal. The ionized molecules tend to be forced toward the exterior boundaries of the ferroelectric crystal by continuing frequent changes in polarization of the ferroelectric material. The ability of ferroelectric material to retain a remnant polarization state diminishes as the lattice structure of the material breaks down due to ionic migration. In normal operation many millions of operations per second may be performed on a memory cell which may switch, and thereby fatigue, the ferroelectric capacitor in a memory cell. If the ferroelectric RAM fatigues quickly, the memory has too short a useful lifetime. For example, each write operation to a memory cell which changes the contents of the memory cell causes a switch in the polarization of the ferroelectric material. In addition, each read operation, as discussed above, may cause the polarization of the ferroelectric capacitor to be switched twice: once to sense the present polarization (by destructively sensing the polarization), and again to restore the polarization. This fatigue of ferroelectric material as used in capacitors of non-volatile RAMs has been a contributing factor in the lack of commercial success for ferroelectric RAM memory devices.